Studies have identified amyloid beta (A?) protein accumulation in the synapse as a neurotoxic hallmark associated with synaptic dysfunction, which occurs early in Alzheimer’s Disease (AD) pathology. Clearance of soluble A? from the synapse is thought to occur through binding of A? to high density lipoprotein (HDL) and apolipoprotein E (apoE) complex in the brain. Studies suggest that the resulting A?/apoE/HDL cholesterol complex is a mechanism of internalizing and clearing A? from the synapse through apoE receptors, low density lipoprotein receptor (LDLR) and low density lipoprotein receptor related protein 1 (LRP1). This clearance process is thought to be dysfunctional in AD, however, evidence regarding specific molecular processes in the synaptic compartment involving apoE, A?, LDLR, LRP1, and cholesterol across AD stage and APOE genotype remains unclear. Therefore, levels of LDLR, LRP1, and cholesterol were measured in synaptosomes across AD stage and APOE genotype using flow cytometry and Western blot analysis.
Experimental results demonstrate that A? and apoE levels were increased in synaptosomes that were postitive for LDLR and LRP1 in late AD cases, which demonstrates possible co-localization of apoE receptors with A? and apoE. Additionally, LDLR was increased while LRP1 was reduced in late AD, suggesting that downregulation of LRP1 may contribute to dysfunction in A? clearance while increases in LDLR in late AD may be associated with a compensatory clearance mechanism for processes that is inefficient. Furthermore, esterified cholesterol was reduced while free cholesterol was increased in late AD versus normals, suggesting that increased lipid storage may play a role in AD prevention and free cholesterol in late AD may be associated with membrane instability and subsequent synaptic loss. APOE3/3 synaptosomes showed increased free cholesterol level and reduced esterified cholesterol level in late AD versus normals. APOE+e4 synaptosomes, however, showed no signficant cholesterol level changes across AD disease stage, implying that dynamic cholesterol turnover is necessary for synaptic health. Overall, the results suggest that apoE receptors and cholesterol level alterations are associated with AD pathogenesis via disruption of A? clearance mechanisms.